Supramolecular Chloride Reservoirs Enable Homogeneous Halide Distribution and Near‐Unity Blue Luminescence in Cs ₄ PbBr ₆ ‐CsPbBr ₃ Heterostructured Perovskites

ABSTRACT Blue‐emitting metal halide perovskites remain difficult to stabilize because mixed‐halide compositions suffer from halide heterogeneity, defect formation, and rapid phase segregation. Here, we report a supramolecular host engineering strategy that stabilizes mixed‐halide perovskites using a quaternary ammonium chloride‐functionalized cationic β‐cyclodextrin (C‐βCD). The cyclodextrin host simultaneously functions as a chlorine reservoir, defect passivator, and supramolecular stabilizer, enabling homogeneous halide incorporation and strong host–guest interactions with the Cs 4 PbBr 6 ‐CsPbBr 3 heterostructured perovskite surface. This cooperative regulation effectively suppresses halide segregation and non‐radiative recombination, yielding tunable blue emission with exceptional color purity and a near‐unity photoluminescence quantum yield (PLQY) of 99.9%, among the highest values reported for blue‐emitting perovskites. The resulting supramolecular perovskite luminescent membranes further exhibit remarkable stability against water exposure (93.3% of initial PL intensity retained after 626 h) and ambient environmental stress (a PLQY half‐life of 27457 h), establishing a new benchmark for environmentally robust blue emitters. Integration of the supramolecular perovskites within porous membranes enables multifunctional operation, including foldable display, white light‐emitting diodes, as well as pioneer application of fluorescence detection and visible‐light‐driven degradation of perfluorinated pollutants. These results highlight supramolecular host–guest chemistry as a powerful molecular strategy for stabilizing mixed‐halide perovskites and engineering robust luminescent materials.